This description relates to retroreflective marker-tracking systems.
Known systems for obtaining coordinates of a point or points of interest include marker-tracking systems. Such marker-tracking systems typically rely on objects having one or more markers affixed thereto. The markers that are affixed to the object may be active markers (e.g., light emitting diode markers), passive markers (e.g., retroreflective markers), or a combination of active and passive markers.
In a medical application context, a user (e.g., a doctor) touches the surface of interest (e.g., a surface of a patient's body) using a distal tip of an object (e.g., a probe or a surgical instrument). A marker sensing device (e.g., a pair of cameras) views the marker(s) affixed to the object. On the basis of the known locations of the cameras and the location of the marker(s) as seen by each camera, such systems calculate the three-dimensional coordinates of the marker(s). Then, on the basis of the known relationship between the location of the marker(s) and the location of the object tip, the marker-tracking system determines the coordinates of the object's tip. With the object's tip on the surface, those coordinates also correspond to the coordinates of the surface at that point.
One example of a passive marker is based on glass bead technology. Such a passive marker (referred to in this description as a “multi-lens high refractive index marker”) is formed by embedding tiny glass beads 902 (e.g., numbered in the hundreds or thousands) in a substrate 904 (a close up of which is shown in FIG. 9), and coating a surface (e.g., a sphere) with the substrate. The highly-textured surface of the sphere is susceptible to contamination from dirt, finger oils, etc. Any contamination that is present on the surface of the sphere may affect the retroreflective performance of the passive marker, thereby contributing or causing inaccuracies in the determination of a position of the object to which the marker is affixed.
True retroreflection connotes a manner of reflection where energy is directed back toward its source. At times, in practice, the observation point (e.g. the marker sensing device) cannot be located in a manner so as to allow observation of true retroreflected energy.